Nuclear Reactor
** It has been possible to control fission of U-235 so that energy is released slowly at a usable rate.
** Controlled fission is carried out in a specially designed plant called a nuclear power reactor or simply nuclear reactor.
** The chief components of a nuclear reactor are:
** The chief components of a nuclear reactor are:
(1) U-235 fuel rods which constitute the ‘fuel core’. The fission of U-235 produces heat energy and neutrons that start the chain reaction.
(2) Moderator which slows down or moderates the neutrons. The most commonly used moderator is ordinary water. Graphite rods are sometimes used. Neutrons slow down by losing energy due to collisions with atoms/molecules of the moderator.
(3) Control rods which control the rate of fission of U-235. These are made of boron-10 or cadmium, that absorbs some of the slowed neutrons.
Thus the chain reaction is prevented from going too fast.
(4) Coolant which cools the fuel core by removing heat produced by fission. Water used in the reactor serves both as moderator and coolant. Heavy water (D2O) is even more efficient than light water.
(5) Concrete shield which protects the operating personnel and environments from destruction in case of leakage of radiation
Light-water Nuclear power plant
** Most commercial power plants today are ‘light-water reactors’.
** In this type of reactor, U 235 fuel rods are submerged in water. Here, water acts as coolant and moderator. The control rods of boron-10 are inserted or removed automatically from spaces in between the fuel rods.
** The heat emitted by fission of U-235 in the fuel core is absorbed by the coolant. The heated coolant (water at 300°C) then goes to the exchanger. Here the coolant transfers heat to sea water which is converted into steam. The steam then turns the turbines, generating electricity. A reactor once started can continue to function and supply power for generations.
**About 15 per cent of consumable electricity in U.S.A. today is provided by light water reactors. India’s first nuclear plant went into operation in 1960 at Tarapur near Mumbai. Another plant has been set up at Narora in Uttar Pradesh.
** While such nuclear power plants will be a boon for our country, they could pose a serious danger to environments. In May 1986, the leakage of radioactive material from the Chernobyl nuclear plant in USSR played havoc with life and property around.
** Disposal of reactor waste poses another hazard. The products of fission e.g., Ba-139 and Kr 92, are themselves radioactive. They emit dangerous radiation for several hundred years. The waste is packed in concrete barrels which are buried deep in the earth or dumped in the sea. But the fear is that any leakage and corrosion of the storage vessels may eventually contaminate the water supplies.
Breeder Reactor
** We have seen that uranium-235 is used as a reactor fuel for producing electricity. But our limited supplies of uranium-235 are predicted to last only for another fifty years. However, nonfissionable uranium-238 is about 100 times more plentiful in nature. This is used as a source of energy in the socalled breeder reactors which can supply energy to the world for 5,000 years or more.
** Here the uranium-235 core is covered with a layer or ‘blanket’ of uranium-238. The neutrons released by the core are absorbed by the blanket of uranium-238. This is then converted to fissionable plutonium-239. It undergoes a chain reaction, producing more neutrons and energy.
** The above reaction sequence produces three neutrons and consumes only two. The excess neutron goes to convert more uranium to plutonium-239. Thus the reactor produces or ‘breeds’ its own fuel and hence its name. Several breeder reactors are now functioning in Europe. However, there is opposition to these reactors because the plutonium so obtained can be used in the dreaded Hbomb.
Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition.
